The present invention relates to a closed-cycle adsorption system wherein a sorbate is alternately adsorbed onto and desorbed from a sorbent in order to cycle the sorbate between a low pressure state and a high pressure state. More particularly, the invention relates to a closed-cycle adsorption system which employs electrical energy to desorb the sorbate from the sorbent in a substantially non-thermal reaction.
In existing adsorption and absorption compression systems, which will be referred to herein simply as sorption compression systems, a first, typically gaseous substance called a sorbate is alternately adsorbed (or absorbed) onto and desorbed from a second, typically solid substance called a sorbent. Particular sorption compression systems utilize specific sorbates and sorbents to produce a desired effect which is dependent on the affinity of the two substances. During the adsorption reaction, the relatively low pressure sorbate is drawn onto and combines with the sorbent to produce a sorbate/sorbent compound. During the desorption reaction, energy is supplied to the sorbate/sorbent compound to break the bonds between the sorbate and sorbent molecules and thereby desorb the sorbate from the sorbent. In this reaction, the sorbate molecules are driven off of the sorbent molecules and into a relatively high pressure, high energy gaseous state. Substantial energy is imparted to the sorbate during the desorption reaction, and this energy can be harnessed for various uses.
A common use of sorption compression systems is in the field of refrigeration. An exemplary sorption compression refrigerator may use a polar refrigerant as the sorbate and a metal halide salt as the sorbent. During the desorption reaction, which occurs in an enclosure called a sorber, the refrigerant molecules are driven off of the salt and into a relatively high pressure gaseous state. The refrigerant gas is subsequently condensed and then evaporated to produce a cooling effect. The evaporated refrigerant gas is then channeled back to the sorber, where it is once again adsorbed onto the salt in an adsorption reaction. The sorption cycle is repeated numerous times depending on the cooling requirements of the refrigeration system.
Though sorption compression systems offer certain advantages over mechanical compressors, their efficiency is generally limited by the characteristics of the desorption and adsorption reactions. For example, prior art heat-activated sorption compression systems require a great deal of thermal energy to stochastically heat the sorbate/sorbent compound to a degree sufficient to break the bonds between the sorbate and sorbent molecules. This thermal energy is commonly supplied by a gas or electric heater whose heat is conducted to the sorbate/sorbent compound through a typically metal sorber. As a result, a substantial amount of sensible heat is added to the sorbate, the sorbent and the sorber during the desorption reaction. However, since the sorbent must usually be relatively cool to adsorb the sorbate molecules, a significant amount of time and/or ancillary cooling means are required to remove this sensible heat and cool the sorber and the sorbent before the next sorption reaction can proceed, and these necessarily reduce the efficiency of the system.
Certain prior art sorption compression systems have been developed to address the problems associated with heat-activated sorption compression systems. For example, the sorption compression refrigeration system described in U.S. Pat. No. 5,842,356, which is commonly owned herewith, uses electromagnetic energy to drive the desorption reaction. The desorption energy is supplied in the form of electromagnetic waves, such as radio frequency waves or microwaves, which are generated by, for example, a magnetron. Instead of heating the sorbate/sorbent compound, the electromagnetic waves selectively pump electrical energy into each sorbate-sorbent bond until the bond is broken and the sorbate molecule is separated from the sorbent molecule. Therefore, the sorbate, sorbent and sorber are not heated during the desorption reaction, and the overall efficiency of the refrigeration system is consequently greatly improved.
However, the efficiency of electromagnetic energy-activated sorption compression systems is nevertheless limited by the types of sorbents which must be used in such systems. These sorption systems must usually employ a sorbent which has a low loss tangent so as not to be heated by the electromagnetic energy during the desorption reaction. Furthermore, in all sorption systems, including electromagnetic energy-activated sorption systems, the kinetic energy of the sorbate molecules is converted to heat as the sorbate molecules combine with the sorbent molecules during the adsorption reaction. This heat, which is often referred to as the heat of adsorption, must be dissipated prior to the next adsorption reaction so that the sorbent can re-adsorb the sorbate. However, since sorbents with low loss tangents tend to be poor thermal conductors, either sufficient time or external cooling means must be provided to cool the sorbent prior to the next adsorption reaction.
In light of the foregoing, a need exists for a sorption compression system which has a simplified structure and a relatively high efficiency.
The inventors have discovered that such a sorption compression system may be realized by positioning a sorbent in an enclosure between first and second electrical conductors, adsorbing a sorbate onto the sorbent to form a sorbate/sorbent compound, conducting an electrical current through the sorbate/sorbent compound to desorb the sorbate from the sorbent, and repeating the adsorption and desorption steps to cycle the sorbate from a relatively low pressure state during the adsorption reaction to a relatively high pressure state during and after the desorption reaction. The sorbate and sorbent materials are ideally chosen such that the sorbate/sorbent compound will not heat appreciably when the electrical current is conducted therethrough. Consequently, the desorption reaction is substantially non-thermal. Furthermore, the sorbent material optimally has a relatively high thermal conductivity so that the heat of adsorption will be readily dissipated from the sorbent after each adsorption reaction.
In accordance with the present invention, therefore, a sorption compression system is provided that comprises an enclosure which includes first and second electrical conductors, a sorbent which is positioned in the enclosure between the first and second conductors, a sorbate which is capable of combining with the sorbent in an adsorption reaction to form a sorbate/sorbent compound, a power supply which is connected to the conductors and which is selectively actuated to generate a current that is conducted through the sorbate/sorbent compound to desorb the sorbate from the sorbent in a desorption reaction, and a pressure chamber which is connected to the enclosure and which receives the sorbate from the enclosure during the desorption reaction and releases the sorbate into the enclosure during the adsorption reaction. The adsorption and desorption reactions are repeated to cycle the sorbate between a low pressure state and a high pressure state. In addition the desorption reaction is substantially non-thermal. Consequently, the sorbent does not have to be cooled after each desorption reaction. In addition, the sorbent preferably has a relatively high thermal conductivity so that the heat of adsorption will be readily dissipated from the sorbent after each adsorption reaction. Therefore, the efficiency of the sorption compression system is greatly improved over prior art sorption compression systems
These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers are used to denote similar elements in the various embodiments.